As is generally known in the art, an electron column is used to generate an electron beam by emitting electrons. A microcolumn is a very small electron column constructed by miniaturizing an electron column, which uses a principle of controlling an electron beam conventionally employed in a CRT, in an electron microscope, for electron beam lithography, and in various electron beam apparatuses.
FIG. 1 illustrates the structure of a microcolumn 100 as a typical very small electron column. The microcolumn 100 includes, as its basic component parts, an electron emitter 110, a source lens 140, deflectors 160, and a focus lens 170.
An electron emitter holder 120 is defined with a through-hole 121 in the center portion thereof, and the electron emitter 110 is inserted into the through-hole 121. The electron emitter holder 120 having the electron emitter 110 inserted therein is concentrically coupled to a holder base 130.
The source lens 140 is coupled to the lower surface 131 of the holder base 130. The source lens 140 can be directly coupled to the lower surface 131 of the holder base 130 by bonding, etc. The holder base 130, having the source lens 140 coupled thereto, is inserted into a column base 150.
The column base 150 has a hollow cylindrical configuration and includes an end portion 151 which has a through-hole (not shown) defined in a center portion thereof so that the electron beam emitted from the electron emitter can pass through the through-hole. The column base 150 has a space in which the holder base 130 is received and coupled to. A plurality of through-holes are radially arranged and formed through the column base 150, and the deflectors 160 are inserted through the through-holes. The focus lens 170 is coupled to the lower end of the end portion 151 of the column base 150 by bonding, etc. Therefore, the vertical position of the focus lens 170 can be determined by the position of the end portion 151.
Generally, in an electron column, in order to allow the electron emitter to stably emit electrons, it is necessary to maintain a high vacuum. A microcolumn, as a typical electron column, must be employed in an ultra high vacuum of about 10−9 Torr due to the characteristics of the electron emitter. The ultra high vacuum is created by using an ion pump or a getter pump. In order to quickly create the ultra high vacuum, bake-out must be conducted.
However, the electron beam generated by the electron column can be sufficiently utilized even in a vacuum of about 10−3 Torr, which is not an ultra high vacuum. That is to say, when the electron beam generated by the electron column is incident on a sample, the chamber or space in which the sample is placed need not be maintained in an ultra high vacuum state, and it is sufficient to maintain the chamber or space in a (normal) vacuum state.
In this regard, if the electron column and the sample are employed in an ultra high vacuum, it takes a long time to change a sample, and the cost of manufacturing the chamber to form the ultra high vacuum increases. That is to say, it is not preferable to maintain the chamber, in which the sample is placed to allow the electron beam emitted from the electron column to be incident thereon, in an ultra high vacuum state, because substantial costs are incurred.
Therefore, when using the electron column, it is necessary to separate the chamber in which the electron column is used and the chamber in which the sample is placed, and to maintain different pressures in the two chambers.